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How can pilots fly inside a cloud?
I heard that sometime an aircraft fly inside a cloud, and I'm wondering how that works - how clearly can the pilot see while inside the cloud?
What if a cloud is "too large"? (Is there a maximum size of cloud a pilot can fly through?)

Maybe a separate question but also you'd want to avoid some sorts of clouds such as storm and ice containing.

When aircraft fly inside clouds, they fly under "instrument rules". It doesn't matter whether the visibility is reduced (at night) or totally blocked (in a thick cloud), this mode of flying simply assumes the crew has no external visual reference, they fly solely using indications given by on-board instruments.
The following short video shows a situation frequently encountered, the crew doesn't see anything outside prior to being so close to the ground, they are nearly landed.

Landing without visibility, source: YouTube.
After the problem of replacing external references has been solved, the next most important issue is to safely stay away from other aircraft. To prevent collisions, pilots are supported by air traffic control (ATC) from the ground. ATC determines aircraft location and altitude by relying on an on-board transponder. ATC issues appropriate information and instructions for pilots in order to maintain aircraft separated.
Should ATC fail to separate aircraft, commercial aircraft are equipped with a safety net: In last resort, close aircraft, transponder-equipped, are detected and avoided by an on-board collision avoidance system. While this system is efficient when there are only 2 or 3 aircraft involved, the escape maneuver must be done quickly and can be therefore sharp enough for a passenger not belted to be injured. This doesn't work if the other aircraft isn't equipped of a transponder, but all commercial aircraft carry such equipment by regulation.
All large airliners we know fly under instrument rules, regardless of the weather, or the time of day. However for a few runways, landing can be done  by a circling maneuver (or circle-to-land maneuver) in which the crew approaches the runway using an instrumental guidance which is interrupted before landing (usually because the guidance is designed for another runway), leaving the crew under visual rules until touchdown (such maneuvers are known to be more hazardous).
For the sake of completeness, a special kind of visual flight, VFR Over The Top, may be allowed over the cloud layer, during the upper part of the flight, where the horizon and other aircraft can be seen.

1. Technical aspects of instrument flying
It's technically possible to fly inside clouds, fog, snow, at night, etc, but this type of flight is regulated, it requires technical elements both in the aircraft and on the ground, and mandatory additional training for the pilot.
Good sense, and regulation, require the pilot to be able, at any time, to:

a/ Maintain an aircraft safe attitude (flight conditions preservation),
b/ Avoid obstacles and other aircraft (collision prevention),
c/ Find their way to a landing aerodrome (navigation).
d/ Know where they are (position awareness),

Flying without visibility basically means knowing how to carry out these tasks without looking outside the cockpit.
A: Maintain a safe attitude
One of the main references a pilot needs is the horizon. When the horizon is invisible, An artificial horizon reproduces the horizontal plane using a gyroscope. This instrument indicates whether the aircraft is pitched or rolled.
Regarding altitude and speed, the same instruments are used both for visual and instrument flying: Barometric altimeter and airspeed indicator.

Main instruments, traditional presentation. Source: Aircraft Spruce
Image above (left-to-right, top-to-bottom):

speed, attitude (artificial horizon), altitude,
turn indicator, heading, vertical speed

Same instruments on the electronic primary flight display of a A330. Source
Being aware of the aircraft attitude without external visual clues is not natural and more difficult than usually expected. There was a famous study (180-degree turn experiment, Bryan, Stonecipher, Aron) in 1954 which showed a pilot not trained to fly with instruments loses control of the aircraft in 3 minutes in average if external references are lost.
B: Collision prevention
As mentioned, a collision prevention service is provided by radio from the ground. An aircraft flying without visibility is separated by air controllers (ATC) from all other aircraft. The exact type of separation provided varies with airspace category, in particular when radar coverage is missing, e.g. when flying over oceans. For EU countries, see Classification of Airspace at SKYbrary.
Civil ATC determines the aircraft location and altitude by interrogating a transponder on-board the aircraft, from the ground. Should the aircraft transponder be not cooperative, ATC might have access to a primary radar, and perform traditional echo measurement, which is less accurate. Primary radar stations are usually operated by military.

Typical ATC room, source: Learn To Fly Here

See also What is a TRACON? for a full description of control rooms in the US.

ATC is definitely a difficult four-dimension task done using 2D displays. Mistakes are made, most of the time they are fixed in time. In the picture below, VRG231 is descending from FL370 while the facing and climbing DCA337 is through FL262. ATC evaluates they will cross safely, but overlooks the closer XCM3018 approaching from the right at FL360 (more details at SKYbrary).

Loss of separation conditions in place: VRG231 is descending. ATC takes DCA337 into account, but overlooks XCM3018, source
Additional on-board equipment can be used for actual nearby aircraft detection. Such collision avoidance system, known as (TCAS or ACAS), detects transponder-equipped aircraft, typically only within a few minutes radius by using a technique similar to ATC interrogation. In addition, TCAS can provide coordinated conflict resolution advisories to the pilots, to increase separation and prevent collision.

B737 TCAS (TA/RA positions on the transponder panel). Source

C: Navigation:
After the two first tasks of the crew (aircraft in safe flight and collision risks under control), it's time to look at how the crew can reach the destination.
Navigational radio aids are positioned at important locations on the ground, and on-board instruments are used to take benefit of them. Today they include VOR (relative bearing determination) and DME (distance determination) for navigation to, from and between airfields. NDB are still used, but their decommissioning has started around the world, they are used as long distance and non-precision VOR.
Extract from Nice airport (LFMN, France) documentation for Runway 04 departures. Note how VOR (circled in green) and NDB (magenta) are used as waypoints. Also note how NIZ VOR-DME is used as a reference for relative bearing and distance (green stars).

Source
Space radio aids, namely GNSS (US GPS, EU Galileo, Russian Glonass...) are complementing or replacing ground aids for operations (ground aids are still used and required by regulation). For example, the same departures from Nice using Precision Area Navigation (P-RNAV) taking advantage of GNSS, inertial and ground aids to obtain a precise combined position:

Waypoints are not referring to any ground aid but defined by their coordinates in the flight management system database. Same source
A manual or an automated landing can be conducted with the guidance of an ILS (instrument landing system), which is a radio lighthouse usable to follow proper direction and proper slope:

ILS principle (magenta clues displayed on the pilot panel)
As you may have noticed, contrary to a persistent belief, pilots don't rely on ATC for navigation (the exception would be when a pilot has lost all instrumental references. ATC may usually be able to provide a position if the transponder is still operational).
In busy terminal airspace, typically around large airports, ATC role is extended. First, the risk of collision being large and airports being in densely populated areas, aircraft must follow more constrained paths, ATC operators constantly monitor these paths and request corrections from pilots when necessary. Second, runways are scarce resources, arriving aircraft (and departing aircraft to a certain extent) must be sequenced (e.g. by adjusting their speed) into orderly and dense landing queues.
D: Position awareness
Current horizontal position has been determined for a long time using VOR and DME and geometry: angle-angle (aka triangulation) or angle-distance equations.
Larger aircraft have also used inertial platforms which are able to provide no only the current position, but also the heading, the ground speed and most uniquely the acceleration, the rate of rotation and the attitude (from which the angle of attack can be obtained).
Today, those technologies are complemented by GNSS which is able to provide the current position and altitude.
The inertial platforms are still used for their complete independence from any external resource, and their precision which is better than GNSS in the short time. Their big con is they drift continuously and must be reset at intervals (e.g. using GNSS data). To take advantage of everything available, sources are often mixed to provide weighted values and cross checking (add to this air sensors providing altitude and airspeed data).

B737 Fight Management Computer page showing current position according to several sensors. Source
Aircraft are also equipped with ground detection, to help prevent so-called "CFIT", controlled flight into terrain. This sensor is based on on-board radar and stored maps and display ground obstacles around. It is used for horizontal monitoring (hill, mountain) as well as vertical monitoring (ground proximity during landing). Such systems are known as EGPWS, enhanced ground proximity warning system.. They warn pilots by loud advisories, e.g. "Terrain! Terrain, pull up!".

VSD/EGPWS display on Boeing aircraft, source

2. Regulation point of view
Visibility conditions are determined to be either visual (good visibility) or instrument (insufficient visibility) and there are two sets of rules when flying (visual and instrument). Flying in instrument conditions requires by regulation to perform the flight under instrument rules.
Flying with reference to instruments only, requires:

The pilot has received the proper training,
The specific equipment is available in the aircraft, and on the ground,
ATC is provided from the ground.

VMC vs. IMC
There is a set of minimum conditions to declare that the outside environment is visible: these conditions are known as Visual Meteorological Conditions (VMC).
When VMC are not achieved, the conditions are said to be IMC, for Instrument Meteorological Conditions.
VMC criteria depends on countries and airspace, though ICAO provides international recommendations, e.g. in France, generally:

Minimum horizontal visibility 5 km (8 km above FL100).
Minimum distance to clouds: Horizontally 1.5 km, vertically 1,000 ft.

VFR vs. IFR
Any flight must be done under one of the two existing sets of rules:

Visual Flight Rules (VFR)
Instrument Flight Rules (IFR).

The rules to be followed are dictated by regulation and are directly dependent on the meteorological conditions.

In VMC, both VFR and IFR flight are allowed.

In IMC, only an IFR flight is allowed, the pilot must be qualified to perform IFR and the aircraft must be certified for IFR.

Related:

Spatial disorientation which may lead to fly upside down without knowing. Related training.

Pilots that fly in clouds knowingly will be under IFR (instrument flight rules) and will have contact with traffic control to keep away from other planes. If you end up in a cloud by accident the standard procedure is to turn around 180° keeping the same height and continue until out of the cloud (or transfer to IFR).
A pilot in a cloud doesn't rely on what he sees outside and instead looks at his instruments.

source wikipedia
They are in order: airspeed display, artificial horizon, altitude display, turn coordinator, heading (compass) and vertical speed.
There is another layout for this information:

With the same-ish layout, airspeed on the left, horizon in the center, altitude on the right and heading at the bottom.

A pilot has no clearer vision through a cloud than you looking out the window at the same time. However, the flight can proceed in safety with a combination of instruments and the facilities available to an air traffic controller.
In order for a pilot to enter a cloud, s/he must be flying under Instrument Flight Rules, which among other things means that an air traffic controller is responsible for separation from other aircraft (contrast with Visual Flight Rules where the pilot him/herself is responsible for seeing and avoiding other aircraft).
In addition, pilots have instruments, such as an artificial horizon, which allows them to maintain any climb/descent and turn required without sight of an actual horizon - the main way a pilot can usually tell if they are climbing, descending or turning.

These are some very well written and complete answers. I would also like to offer my own perspective and context in the matter. A modern IFR aircraft will have 2 sets of flight instruments: (1) primary, and (2) secondary, and these are significantly different. This is an important point not to be overlooked. It is emphasized in training. We are very fortunate with today's technology, and this always hasn't been the case.
As a US Navy pilot we spent hours in simulators practicing IFR procedures, while handling emergencies. I want to stress that these flights were designed to help us focus on 2 important aspects: (1) flight in clouds, or other low visibility conditions, while (2) successfully handling emergencies in this challenging environment. There are a couple of other finer points I would like to make.
We might not think of it, but one can be flying VFR without a horizon, and in this case a pilot is doing a little of both. I spent a lot of time flying over the Mediterranean. Particularly during the summer months, where the haze and sea blended together, allowing the horizon to disappear. I remember this being particularly true above 5,000 ft AGL. During these months, even a starlit night could become disorienting. The lights of ships on the water could appear as stars to the pilot, which then altered where the horizon was in their mind's eye.
Even with our modern navigational systems IFR flight can be very difficult, even for someone with a lot of experience. On one such Mediterranean night described above the lead of section became disoriented and began a slow descending spiral. It can take a lot of discipline to believe in what your instruments are telling you, when your body is screaming something else at you. At times the body wins. Even with his wingman urging him to level his wings, the pilot ended up flying into the sea.
The simulators helped us practice to rely on the instruments, and at the same time deal with the distractions of various cockpit emergencies. The best simulator I had was well planned and executed by the Wizard of Oz. He was  running the simulator controls. It started with a slight flicker of the oil gauge at start up, ran into deteriorating weather airborne, with more engine problems, and a partial electrical failure. Eventually, I was reduced to using pressure instruments.
The navigation system I flew with was called an Inertial Navigation Systems (INS), and it got its input from gyroscopes that maintained axis orientation from their rotational motion. The primary attitude indicator was very responsive, with no perceptible lag time between changes in flight path and response from the INS. With a good primary attitude indicator, and other non-pressure sensitive instruments, e.g. radar altimeter, it is relatively easy to maintain controlled flight. If the INS should fail though, that was a whole other ball game.
With an INS failure, we were left with the secondary instruments. This cluster was comprised of a small standby attitude indicator, and the following pressure instruments: altimeter, vertical speed indicator (VSI), and airspeed indicator. Finally, there was the turn needle and standby compass. Flying on pressure instruments in IFR conditions is very challenging because of the significant lag between what the instruments are displaying and the actual flight path of the aircraft. The VSI was the most sensitive, and the altitude indicator was the least sensitive. One could easily find themselves "chasing" their needles in a fight to control the negative feedback.
So there are primary flight instruments and secondary flight instruments. With the high reliability of today's avionics systems we thankfully don't have to spend much time on secondary instruments.

In the middle of the instruments is the large primary attitude in indicator, and below it the compass. The standby compass is difficult to see, but is just above the glare shield on the right-hand side. At around 7 to 8 o'clock directly to the left of the primary attitude indicator is the standby attitude indicator. Above that is the mach/airspeed indicator, the pressure altimeter, and at the top the radar altimeter. Just to the left of those instruments, and slightly smaller, you can make out from top to bottom, the angle-of-attack indicator, VSI, and accelerometer.
And so I found myself in a Ground Controlled Approach at my bingo field, on secondary flight instruments, with a faltering engine, at minimums. At around 800 feet the Wizard of Oz ordered a fire warning light, followed shortly after with a catastrophic engine failure. I didn't get to the ejection handle quick enough.
At the time I had a neighbor who had been a pilot in World War I. We were sitting around and I was telling him about the simulator flight, jokingly complaining about how one-by-one he failed instruments on me, when he stopped me with his laugh and said, "Son, when we found ourselves in a cloud we flew with one hand gently holding up a pencil in front of our face in the open cockpit, and the other hand holding onto the stick."

re last paragraph -- sounds essentially like the principle noted here -- https://wrightstories.com/wrights-develop-automatic-stabilizer/

In accounts of World War I aviation, we sometimes read of pilots operating in clouds for extended periods.  It strains credulity to think that this was actually possible with the primitive instrumentation of the time.
It is very difficult to maintain control of an airplane or glider in cloud without at least one gyroscopic instrument to give an indication of whether the aircraft is wings-level or banked.  Bear in mind that losing control is more than just a navigational issue-- it is very easy to overstress an aircraft and cause it to break apart by accidentally entering a steep turn or dive in a cloud.
While most modern airplanes have an artificial horizon (attitude indicator) instrument, it is possible to maintain control of an aircraft in cloud using a turn rate indicator and no other gyroscopic instrument.  In modern terms, this is called "partial panel" flying.
The first gyroscopic turn rate indicator was created in 1917.  Charles Lindbergh flew his Ryan NYP aircraft "Spirit of St. Louis" across the Atlantic Ocean in 1927, and was in cloud for extended periods, with a turn rate indicator as his only gyroscopic instrument.  The Ryan NYP also had an "earth inductor compass", which provides superior performance in flight to a standard magnetic compass.  Jimmy Doolittle was one of the pioneers of blind flying, making the first flight that was completely blind from takeoff to landing in 1929.
Sailplane pilots have often flown in clouds for extended periods using a turn rate indicator as the only gyroscopic instrument.  There are some specialized magnetic compasses designed for partial-panel cloud flying in gliders that have reduced succeptibility to the well-known errors that more conventional magnetic compasses suffer from in turning flight.  One such compass is the Cook compass, which may be manually adjusted to match the bank angle that the pilot is intending to maintain in the turn.  Another such compass is the Bohli compass, the needle of which is designed to exist in full three-dimensional alignment with the earth's magnetic field, so that turning errors are nearly completely eliminated as the glider and compass housing rotate freely around the needle.  The Bohli compass is designed to provide the same information as an artificial horizon (attitude indicator), albeit in a manner that is much less intuitive to interpret.  Also, the lack of turning errors allows the pilot to use the heading information from the compass to systematically make adjustments to his or her thermal circles to center the best part of the lift while circling in cloud.
Link to pdf of "Air Force" magazine article on Doolittle's pioneering blind flight in 1929
Link to pdf for manual for Bohli compass
Image of Bohli compass:

As mentioned by other posters flying within and through clouds is considered to be instrument meteorological conditions (IMC), that is, where flight is accomplished solely by reference to instruments.  The flight must be conducted in accordance with instrument flight rules (IFR).  In controlled airspace, this requires filing an IFR flight plan and receiving a clearance to fly it from Air Traffic Control (ATC).  While flying an IFR flight plan, you will remain in constant radio contact with ATC facilities while airborne in controlled airspace for traffic separation.
Clouds in and of themselves are not hazardous to fly through but may contain hazardous weather within them such as embedded Thunderstorms/Cumulonimbus, icing, and turbulence.  Sometimes exterior shapes of clouds such as towering cumulonimbus indicate thunderstorms ahead or lenticular altocumulus can indicate severe turbulence in or nearby.  It is required by law that a pilot obtain a weather briefing prior to an IFR flight to determine weather conditions enroute and in the terminal environment to better prepare a flight plan and make ones self aware of meteorological hazards.
While flight may be made through clouds safely, approaches and landings may not, with very few exceptions, as previously discussed.  Instrument approaches have specific ceiling and visibility minimums which have to be observed.  If a pilot cannot see the runway environment which meets the published visibility minimums at the missed approach point or decision altitude, they must abort the landing attempt and fly the appropriate missed approach procedures for that approach.  Only specially trained aircrews flying specifically equipped aircraft into airports equipped to handle coupled autopilot landings on specific instrument approach procedures may land in zero visibility conditions.


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Does a safety wire prevent a fuel filter to detach during flight?
I know that fuel filters on aircraft engines have some features that avoid the bowl from detachment during flight, and that they can be secured using safety wire as shown below.

I've also heard that installing safety wire is very time consuming. Is that true?
What is your experience with installation & safetying of fuel filter bowls?

Here is a video describing installation of lockwire (safety wire) in general. The most common use indirect; the lockwire holds the screws so they can't shake loose.

@JanHudec That's essentially how they're used on the fuel strainers too - the bail nut is what's holding the bowl in place, and the wires hold the bail nut (and the bail) so it can't shift in flight and let the bowl fall off.

Safety wire is almost never "efficient", but efficiency is not the goal: Safety is.
You want to be certain that once the fuel strainer has been secured it's not going to fall off in flight (which will result in the engine being starved of fuel).
On many light aircraft that's accomplished by wrapping safety wire on the fuel strainer to ensure it can't come loose in flight:

A skilled mechanic who works with safety wire every day can safety a fuel strainer in about a minute - probably even faster. It's pretty impressive to watch.
A bungling incompetent like me who does it maybe once a year takes about 5 minutes, often punctuated with "colorful metaphors" and comments about the questionable parentage of the engineer responsible for the fuel strainer.
Either way, in the grand scheme of things that's not a lot of time spent on securing a flight-critical component.

Because there are far more bungling incompetents than skilled mechanics (and because even good mechanics are frequently not fond of safety wire) there are other fuel strainer designs which don't use safety wire.
The one I linked to uses a bayonet lock for the bowl, and a cotter pin to keep it from coming loose in flight. Anyone can bend a cotter pin in a couple of seconds, and as effective as the safety wire technique, if not moreso.

@Muzza Yeah, I don't know anyone who twists safety wire by hand, except for the first twist or two after a fastener. It's almost impossible to get a good uniform twist without using safety wire pliers, and if the twist is sloppy you've got to cut the wire off and start over…

I have seen some filters that have a ratchet mechanism to secure the filter bowl instead of safety wire and are supposed to be easier to remove and install. Are they really better than safety wire? what is your experience with these type of mechanisms?

@Aerochar I've never seen a "ratchet" mechanism so I'm not entirely sure what you're referring to. The bayonet-lock one I linked to is kind of like what you find on modern SLR camera lenses (line up two tabs and rotate about 30 degrees until it stops in position) except there's no locking tab like most camera lenses have: That functionality is accomplished with the cotter pin. Personally I like that idea better than safety wire as it seems less error prone (like I said, anyone can bend a cotter pin - twisting safety wire is an art form).

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What are the requirement for electrical grounding of aircraft in hangar storage?
When an aircraft is being stored in a hangar between flights, are there any requirements to have the aircraft connected to an earth ground?
I believe that there is (although I'm looking for the reference), and if so, why and how should this be accomplished?  Is attaching a cable to the metal structure of the hangar an accepted practice?

Interesting question. In my time in the Marines we never grounded a single aircraft parked inside our hangars.

In general any aircraft parked in hangar should be electrically grounded. The grounding should be done from basic aircraft structure to a low resistance ground.
Grounding of the aircraft is usually done to,

Protect aircraft and personnel against hazards from lightning discharge.
Provide current return paths
Protect personnel from shock hazard
Prevent accumulation of static charge

During de-fueling or refueling, the aircraft is grounded to the fuel truck.
In general, the aircraft are grounded to a point where the impedance is less than 10,000 ohms referenced to earth for static grounding. For power grounding, it should be less than 10 ohms to power systems neutral.
Aircraft are not grounded to the hangar structure as the external power supply, when connected, is grounded to that.
The best reference, if you ask me, would MIL- HDBK-274(AS)- Electrical Grounding for Aircraft Safety.

Lightning inside a hangar? Wouldn't the structure protect the aircraft from that?  Also, I'm mainly looking to see if there is a requirement to do this, or if this is just a commonly practice.

Actually, lightning would be a good reason to not ground the craft to the hangar itself. If the lightning hit the hangar, it would go direct to ground, if the craft is tethered to it (electrically) some power will flow into the aircraft and potentially cause damage. (Which sounds like a good question on its own, but most likely open to speculative answers only.)

@FreeMan I just had that conversation with someone yesterday!  I would think that it wouldn't cause any damage in this case because the path to ground via the hangar would be much lower resistance than the path through the aircraft tires to ground and therefore wouldn't flow to the aircraft.  That's just my "common sense" theory though, not based on anything substantial.  :)

A minor nitpick: When you connect an aircraft to a refueling truck (or a deicing truck) you're not grounding it - you are bonding the two structures electrically so they're at the same potential. (This is important, because if something connected to earth ground comes close enough to the two vehicles it could still generate a spark.)

I have seen a lot of aircraft in a lot of hangars in several countries and never once seen one grounded or bonded (except perhaps incidentally if hooked up to a battery tender).

NFPA (National Fire Protection Agency) 409 - Standard for Aircraft Hangars states in each chapter that deals with the 4 types of Hangars:

8.6.1*     Grounding facilities shall be provided for removal and control of static electrical accumulations on aircraft while aircraft
  are stored or are undergoing servicing in a hangar.
8.6.2     Floor-grounding receptacles shall be provided and shall be either grounded through individual driven electrodes or electrically
  bonded together in a grid system and the entire system grounded to
  underground metal piping or driven electrodes. Where driven electrodes
  are used, they shall consist of 15.9 mm ( 5/8 in.) diameter or larger
  metal rods driven at least 1.5 m (5 ft) into the ground.
  Floor-grounding receptacles shall be designed to minimize the tripping
  hazard.
8.6.3*     Grounding wires shall be bare and of a gauge that will be satisfactorily durable to withstand mechanical strains and usage.

So it is not a law per se, unless one has adopted a code that references NFPA 409, but at the very least it is a recognized standard for best practices that will be used in a court of law after an incident.

There is a requirement to have the aircraft grounded to an approved grounding point in the hangar under certain conditions. This is for a number of reasons including the skin of the aircraft is the return path as we only use one wire in aircraft circuits and the return path is the aircraft skin itself.
Also the grounding points must be approved. When the tarmac or hangar is built the grounding bars or points are tested to make sure they are within specs. The resistance must be low enough to ensure they are the path of least resistance.
The first thing we do when an airplane enters the hangar is ground it. Every spot usually has its own grounding spot.
There are times that the airplane is not grounded, but typically for maintenance it is. Maintenance usually requires ground power to be applied to the airplane. Also when large aircraft and especially helicopters return from flight, they sometimes can have a large buildup of static electricity and this has caused some serious injuries in the past. This static electricity has to be dissipated and is done via a grounding wire to an approved grounding point.

AC21.99 Advisory Circular
  Electrical bonding of aircraft to earth has generally
  been aimed at protecting aircraft and personnel from the
  hazards associated with static electrical discharge.
  However, with utilisation of external power sources,
  electrical bonding to earth must also protect aircraft and
  personnel from the potential hazards associated with the
  electrical ground power supplies.

https://www.casa.gov.au/sites/g/files/net351/f/_assets/main/rules/1998casr/021/021c99s2c14.pdf

In our hangars, there are no grounding points like you describe, so it isn't in all hangars that have been built. You say that there is a requirement: Is it a FAR, federal law, or company policy requirement?  That's mainly what I'm looking for: Is there an actual requirement for this, or just common practice?

I've hangared Robinson R22 and Westland Wessex between flights and neither had statutory requirements, one CAA, one RAF, concerning fuel or earthing.  If they are entering for servicing, then that's another matter.

I've been in a lot of hangars, from simple T style private hangars to large airline maintenance hangars.  Not one of them had built in grounding points, and not one of the airplanes in them were grounded.

@ralgha I've found a lot of military references to this (procedures, etc.) but have a similar experience to yours in civil aircraft hangars.

@Lnafziger I added the Advisory circular references for you. While it is from the Australian site we use it in Canada and the US. In the military we always ground the aircraft (fighters, helicopters, large transport) for safety reasons. I have seen lots of aircraft ungrounded but never during refuelling or maintenance with power applied.

The Australian AC21.99 is a good example of a "reticulated" system; one with a central source, and transformer isolation at each service point. This is speaking to 400HZ service only. But I do not find a requirement in the US NEC, or FAA that talk about this. The old MIL-HDBK-274 had a lot of good information, especially in defining "power" grounds, and "static" grounds. I designed several Navy Hangar power systems, and before I retired, I inspected 83 or the US DoD hangars. My interest began when I did an investigation on a damaged F/A-18D, which was attributed to a faulty hangar power service station. The overvoltage damaged 7 of the Avionics WRAs that had to be replaced. Later, in a Texas hangar I found an improper 400HZ system bond which resulted in 31 volts (p-p) on the 400HZ neutral conductor. Since the aircraft generally use the structure as a neutral conductor and as a "ground" reference, it raises Cain with the electronics. Furthermore, the US Military does not have an inspection process in place for the ground power port, nor do they have a requirement to test power cart and service point cable heads for wear. When a pin becomes loose, the neutral may surge to the missing RMS phase voltage. This then puts 120V, 400HZ on the AC skin, damaging Avionics and shocking/burning maintenance people. Sorry, I can't speak to 28VDC systems. The only commercial reference I have to AC power requirements is the one from ATAA (Air Transport Association of America). Its copyrighted, 1980, but available. The title is "400Hz Fixed Power Systems Design Guidebook", author Ralph Morrison, McCormick-Morgan power systems engineers. Chapter 8 recommends system bonding in excess of NEC requirements due to the impedance effects of 400hz. Sorry for writing a book.

I know of no specific requirements for grounding an aircraft for "storage" within a hangar. However, there are mandatory requirements that address electrical system grounding, and static grounding. These are two separate issues. In the first, the National Electrical Code (NEC), Article 513.10 Special Equipment, (A), Aircraft Electrical Systems,(C), External Power Systems for Energizing Aircraft, (3), Cords; require that the Flexible cords used to power the aircraft "shall include an equipment grounding conductor". DC power system pendant cord plugs contain two contact pins, a + V and  - V, the - V conductor is usually grounded at the power source; but this does not satisfy the Article 513 requirement above. In the case of 3-phase 400-HZ connectors, 6 active pins are provided in the standard connector; Pins for phases A, B, C, and the N neutral, as well as an "E" and "F" interlock circuit. The "N: Neutral pin must be bonded to building steel at the source. As noted above, the aircraft structure is used as a "return" path for both DC, and AC returns.
  Static grounding can be satisfied by the use of a static grounding strap. This is mandatory for US Military aircraft. I have no data on commercial or private manufacturer's requirements.
In the past, there have been horrific hangar fires, such as the Sabena Hangar fire in Belgium in May of 2006. Issues that need to be addressed are: compliance with NEC, effects or multiple voltage systems attached to parked aircraft.
Before I retired, I addressed these issues in an article for EC&M Magazine; see at https://www.ecmweb.com/design/electrical-equipment-issues-parked-aircraft

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Where can the ICAO recommendations be found?
ICAO issues recommendations/rules/statutes/something? that standardizes the regulations for international air travel world-wide.
What are these "rules" officially called and where can they be found?  Are they available on the internet?

ICAO publishes and maintains 19 Annexes to the Chicago Convention. They pertain to all matters in aviation and provide the framework for national legislation in each of the member countries in the form of Standards and Recommended Practices (SARP).
In addition to the 19 Annexes, ICAO also produces more detailed guidance in the form of Procedures for Air Navigation Services (PANS): PANS-ABC, PANS-OPS, PANS-ATM. These are also usually referenced by their Doc number: 2007, 8168, 4444, etc
In addition to the PANS, for specific regions of the world ICAO publishes Regional Supplementary Procedures (SUPP) to incorporate regional variants/requirements.

Any ICAO contracting state may have local legislation different from SARP/PANS, but is obliged to notify ICAO of these differences. You will find these differences in the country's AIP section GEN.
Getting electronic copies of these documents is a bit more difficult.  ICAO sells these documents for a very 'aviation' price. Some CAA's and institutions will provide electronic copies in PDF form, but they will most likely be out of date and their legality is suspicious at best. Welcome to the world of international law.


text:
What is a "Descend Via" clearance?
When flying to Teterboro, NJ (KTEB) and cleared via the JAIKE THREE ARRIVAL from Flat Rock (FAK):

What does ATC expect when they give a clearance like:

N1234, Descend VIA the JAIKE THREE ARRIVAL

Then, what if they give the next clearance when over JAIKE:

N1234, Descend and maintain 7000

I believe that there is a difference in ATC procedures over whether or not the crossing restrictions still need to be made between US and Canadian rules, what should be done in both cases?  (And out of curiosity, does ICAO have anything to say about this situation?)

The "DESCEND VIA" clearance is described in FAA order 7110.65U (pdf) Section 4-5-7 paragraph h, which defines:

h. Instructions to vertically navigate on a STAR/RNAV STAR/FMSP with published >restrictions.
PHRASEOLOGY-
  DESCEND VIA (STAR/RNAV STAR/FMSP name and number)
  TERMINAL: DESCEND VIA (STAR/RNAV STAR/FMSP name and number and runway number).
EXAMPLE-
  “Descend via the Mudde One Arrival.”
  “Cross JCT at flight level two four zero, then descend via the Coast Two >Arrival.”
  TERMINAL: “Descend via the Lendy One Arrival, Runway 22 left.”

This instruction authorizes the pilot to navigate the STAR laterally as published and to navigate vertically to meet all published altitude restrictions.

Clearance to “descend via” authorizes pilots:

To vertically and laterally navigate on a STAR/RNAV STAR/FMSP.
When cleared to a waypoint depicted on a STAR/RNAV STAR/FMSP, to descend from a previously assigned altitude at pilot's discretion to the altitude depicted for that waypoint, and once established on the depicted arrival, to navigate laterally and vertically to meet all published restrictions. ATC is responsible for obstacle clearance when issuing a “descend via” clearance from a previously assigned altitude.

Though speed restrictions are not specifically addressed in the above paragraph, it does note that you will meet "all published restrictions" to include speed restrictions.  This is supported by a later note:

EXAMPLE-
  “United 454 descend via the Haris One Arrival, except after Bruno, maintain one zero thousand.”
NOTE-
  The aircraft should track laterally and vertically on the Haris One Arrival and should descend so as to comply with all speed and altitude restrictions until reaching Bruno and then maintain 10,000. Upon reaching 10,000, aircraft should maintain 10,000 until cleared by ATC to continue to descend.

This example and note pertain to modifications to a published altitude, but the note establishes that you comply with all published altitudes and speeds when flying the procedure as published.
Furthermore, InFO 12014 explicitly states:

Published speed restrictions are always mandatory. Published speed must be complied with regardless of the use of “Climb/Descend Via” clearance.

For the example of the JAIKE 3 Arrival, the clearance

N1234, Descend VIA the JAIKE THREE ARRIVAL

will be issued if you are already established on the arrival laterally and authorizes you do follow the published route and altitudes.  Assuming the FAK transition is your cleared route when you receive the descend via clearance, you would fly the lateral route

FAK QBEES VAALI PALEO PEEDS FUBRR BUZIE SPNCR JAIKE ILENE WACKI REGLE SBJ

and you would descend at your discretion to be at 13,000 ft at JAIKE, at 13,000 ft at ILENE, at 11,000 ft at WACKI (and at 250 kts) and 7000 ft at REGLE.  You descend to and cross the fixes at these altitudes because you have been authorized to vertically navigate the STAR via the "DESCEND VIA" clearance.
NOTE: This arrival contains an "EXPECT" altitude and the controller should not issue a DESCEND VIA clearance until you are past this fix unless you gives you specific instructions as in:

N1234 cross PEEDS at FL240 then descend via the JAIKE 3 arrival

If the controller at any time issues an instruction e.g.

N1234, Descend and maintain 7000

then:
In the USA
then you would continue to navigate laterally on the STAR but would descend to 7000 ft, disregarding any published altitudes on the STAR.  You are no longer authorized to navigate vertically on the STAR.  If the controller requires you to still cross certain fixes at certain altitudes, they will specify those crossing restrictions in the descent clerance, e.g.

N1234 Cross WACKI at 11000 250 kts then descend and maintain 7000 ft.

In Cadanda / ICAO rules
you shall continue to meet all published crossing altitudes on your descent to your cleared altitude.  For reference on this difference, see FAA InFO 12003


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Does an FAA pilot certificate qualify as an ICAO pilot licence?
What is an ICAO pilot license?
I've seen it as a job requirement before, does my FAA ATP count?

ICAO doesn't issue licenses directly, which means this is pretty broad (an ICAO license is issued by a member nation). Is that what you mean? Or do you mean a specific country?

My understanding is that most FAA licenses would also qualify as "ICAO Licenses" (certainly the ATP seems to as we have a bunch of US/FAA certificated ATPs acting as international captains for US airlines flying to/from ICAO member states). From a quick read I think anything from Private up meets the ICAO Annex 1 (personnel licensing) requirements. (It appears that ICAO charges for Annex 1, but the fine folks in the Republic of Serbia have made the 2011 edition available on their Civil Aviation Directorate's website for us to all read and enjoy).

In addition to the Annex 1 requirements, ICAO's FAQ has the following to say about licensing:

ICAO licence or international licence
ICAO does not issue any licences. Licences issued by ICAO Contracting States on the basis of Standards and Recommended Practices of Annex 1 – Personnel Licensing, are habitually called ICAO licences. This has led many to believe that there is a specific ICAO or international licence. The fact is that there is not one single international licence issued by ICAO or any other organization. States issue their own licences based on national regulations in conformity with Annex 1 specifications and validate licences issued by other Contracting States on the basis of bilateral or multilateral agreements or the fulfilment of nationally legislated requirements.

and

International recognition of flight crew licences
The Convention on International Civil Aviation, often called the Chicago Convention, provides for worldwide recognition of flight crew licences issued by any member State of the International Civil Aviation Organization (ICAO) provided that:

the licence meets or exceeds the ICAO licensing Standards of Annex 1 – Personnel Licensing to the Convention on International Civil Aviation; and
the licence is used on an aircraft which is registered in the State which has issued or validated the licence.

If the licence is to be used on an aircraft which is not registered in the issuing State, the licence holder must obtain a validation of the licence from the State of Registry or alternatively obtain a new licence issued by the State of Registry.

So basically your US/FAA license is a valid "ICAO License" as long as you're flying a US-registered aircraft & the nation you're flying to/through accepts US/FAA licenses (possibly with some additional requirements on a per-country basis).
If you got a job for a Serbian airline and needed to fly an aircraft registered in Serbia you would need the Serbian Civil Aviation Directorate to issue you a license (or validate your FAA license) because they are the state of registry.
From what I've read you can get a private license issued pretty easily in most ICAO member states - the procedures are roughly analogous to US FAR 61.75 - but if you want anything more than that there are other hoops to jump through to "convert" your license, which can range from just a written exam to a full-fledged checkride or more -- this is probably what people are referring to when they say it's difficult to get an "ICAO License" issued.

I'm really not picking on Serbia for any reason except they were the first country Google found for me which was nice enough to provide a copy of ICAO Annex 1. That's got to be worth a little search engine candy.


text:
Do departure procedure speed restrictions override airspace speed restrictions?
This is a speed restriction found on the BNGOS 3 RNAV Departure Procedure out of KOPF:

SPEED: ACCELERATE TO 250 KT, IF UNABLE, ADVISE ATC

Opa Locka airport is Class D and is also below the Miami Class B airspace, so 14 CFR 91.117(b) & (c) would apply and we would be limited to 200 KIAS.
My questions are:

When should we accelerate to 250 KT?  I.e does the DP speed restriction override the 91.117 restrictions, and if not, do we need to advise ATC?
Are we expected to maintain 250 KT or is this considered a minimum speed (above 10,000 ft.)?

It seems like ATC wants us to accelerate asap (in fact, the old procedure used to say to accelerate “as rapidly as feasible” or something similar), but I don’t believe that we can until we either enter the Class B or leave the lateral limits.

That's been a long time, good to see you visiting. Welcome back!

@DeltaLima Yes, it has!  I’ve always lurked some and never really completely left, but life kept me too busy for too long, lol.

CFR 91.117 part b) allows ATC leeway. c) seems pretty airtight.  But d) seems to allow some discretion, especially if ATC is informed.  Class D requires two-way communication anyways.

You should remain below 200 knots while under class B. Accelerate to 250 knots once you leave the lateral boundaries of class B or enter the class B, whichever comes first.
The 91.117(b) restriction includes the language "Unless otherwise authorized or required by ATC," but the 91.117(c) restriction does not. You may not accelerate above 200 knots below class B airspace, regardless of what ATC says.
91.117(d) always applies- if the aircraft you are flying can't climb safely at 200 knots then you should fly at your minimum safe climb airspeed.
Based on the wording, I wouldn't necessarily interpret this as a maximum speed. But of course you need to maintain 250 until above 10000 feet.

Opa Locka is described on the website as a "reliever to Miami International Airport".  This would indicate that, under local rules, it might share speed limits for  jets.  Both airports share the same departure frequency 119.45.  It would be understandable that ATC would want the same speed for departing jets for safer spacing.

@RobertDiGiovanni It's understandable but the regulation is clear that ATC doesn't have authority to waive 91.117(c) speed limits.

I agree.  But d) may apply for a local situation, especially since Opa Locka is essentially a satellite of Miami Int.  Sort of where business jets can avoid some of the congestion of the main hub.  Worth looking into, but if ATC (and the plate) says 250 knots, what is one to do?  (In controlled airspace).

@RobertDiGiovanni Same thing as if a controller told you to maintain 300 knots below 10000 or gave you VFR vectors into class B without a clearance. "Unable." I'm not really convinced that 250 is the "minimum safe airspeed" per 91.117(d), particularly since it just says to notify ATC if you can't accelerate to 250.

Generally, one follows orders and then reports it, unless the craft is in danger.  I wonder if anyone reported it already, and what the response was.

@Chris Regarding your last paragraph, should we assume that it's a minimum speed then?  This is the problem that I have with the wording.  It's ambiguous about what to do, and I'm looking for clear guidance on what they actually expect.

@chris Also, are we supposed to "ADVISE ATC" (per the restriction) that we are unable to accelerate to 250 KT due to the Class B overhead?  I wish that the procedure designers would just say "...as soon as practical after leaving the lateral limits of, or upon entering the Class B" or something similar.  Then there's no ambiguity (although one could argue that there isn't any ambiguity because there's a regulation that covers it, I suppose....)

@Lnafziger Agree it's unclear. I suspect it's because the same language is used for KMIA's BNGOS3 departure, where you start out in class B so there's no ambiguity. Might be worth sending the FAA a message to clarify.

@Lnafziger I would interpret it as a minimum speed. Which combined with 91.117(a) makes it a mandatory speed below 10000. The BNGOS4 departure that just finished its comment period changes the language to "ACCELERATE TO 250 KIAS UNTIL LEAVING 10000", which is clearer.

Thanks, the BBGOS4 language is much clearer as far as how long the speed is applicable, and implies that other people found it ambiguous too!

Also, since the questions asks the more generic questions about "airspace restrictions" vs DP restrictions, what if this was at a different airport that didn't underlie Class B?  Would the DP require that you accelerate to 250 within the Class D?  I.e. would it now be authorized in accordance with 91.117, or is that a different mechanism/ATC procedure?

@Lnafziger Yes, I would interpret that as authorization. Though asking and specifically being told to fly 250 knots would be even better.

If you would like to incorporate these comments into your answer to make it more complete, I'll go ahead and accept it (and we can get rid of the comments)!

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why does the NTSB release transcripts of Cockpit Voice Recordings, but not the actual recordings?
From what I have read it is illegal for the National Transportation Safety Board to release Cockpit Voice Recordings, while they are allowed to release its transcripts.
From AirDisaster:

It is illegal for the National Transportation Safety Board, who regulates these recordings, to release them to the public.

From NTSB:

The transcript, containing all pertinent portions of the recording, can be released to the public at the time of the Safety Board's public hearing.

Why? What's the issue with releasing Cockpit Voice Recordings?

probably some arcane regulation about recording in a "private" work environment that is not allowed to be released without consent of who is in the recording

@ratchetfreak But according to http://www.airdisaster.com/cvr/ : "The airlines, who own the original recording, are legally allowed to release it if they so choose. ". So apparently the consent of who is in the recording is not required.

or they got permission in the pilot's contract

@ratchetfreak Good point!

@ratchetfreak They're not in private. They're in a plane owned (or leased or whatever) by their employer, fulfilling their duties as employees. In any given juridsiction there may be rules against secretly recording your employees but this isn't secret recording.

Crew may discuss very embarrassing topics about other people: "My brother Jim told me that he had a gay affair with governor Brown for six months... oops what was that?..." Do we release this recording because at the same time we listen in the background a sound that is critical to the current investigation?

From NTSB site, regarding FOIA requests:

"Cockpit Voice Recorder (CVR) tapes. Title 49 U.S.C. § 1114(c)
  prohibits the release of any CVR tape. However, the NTSB may release a
  CVR transcript (edited or unedited), in accordance with 49 U.S.C. §
  1114(c)(1)."

Transcripts may be disclosed, but not 'tapes'. Which makes sense, as transcripts may be edited to remove parts that are not meaningful for the investigation, but can be  unnecessary intrusions into private life.
In addition, when the NTSB is participating to foreign investigations:

Records or information relating to the NTSB's participation in foreign
  aircraft accident investigations. 49 U.S.C. § 1114(f) prohibits the
  release of this information before the country conducting the
  investigation releases its report or 2 years following the accident,
  whichever occurs first

See 49 U.S. Code § 1114 - Disclosure, availability, and use of information

Thanks. Privacy issues are what I thought at first, but I'm surprised that they can't edit the voice recordings as well (just blanking any private discussions of no interest for the public). IMHO transcribing is a quite lossy process: much information such as the intonations, voice intelligibility, emotions, etc. cannot be well transcribed.

What I think, but this is just an opinion, is that the tone of the voice, the delivery, the breathing difficulties, etc, are intimate information about the individual, which may be not meaningful for the investigation. In that case, it is not necessary to inform the public about crew feelings that can be misunderstood by uninformed or disrespectful persons (this is where it may become private life intrusion). Transcripts only seems good.

I think @mins has a great point: the Apollo 1 recordings were reportedly hard to listen to, and the fairly recent KFDK collision ATC recordings were taken down by many sites. Those might be extreme examples, but the point that some things just don't need a wider audience out of respect is a good one.

They're also extraordinarily harrowing. If you ever do read the transcripts, try and imagine them taking place between two or more pilots trying to deal with a deadly situation for themselves and their passengers. While accompanied by the background noise of violent maneuvering, alarms or engines screaming or indeed engines on fire or exploding. Or in really bad cases, hijackers yelling commands. Or if things are really bad, all of the above. And then often finishing with the sound of a plane impacting the ground, cutting off suddenly and being replaced by an ominous silence. I personally would not want to hear such a recording.
I've seen a transcript which ends with one of the pilots yelling "I love you mom!" just before their plane hits the ground, killing everyone on board...
Finally on a more practical note, pilots use a lot of jargon and abbreviations, some of which is specific to the model of aircraft being flown. If you don't know what a flight level is, or what ATC or approach are, or which check-list they're talking about, then they can sound and read like gibberish. And that's assuming they're speaking English. Flight crews talk in their native tongues whilst flying and not talking on the radio. So for example a recording from an Air France flight would be largely in French.

The aviation jargon would be just as jargony in a transcript...

Any links out there of actual recordings

+1 As an avionics tech, I heard CVR recordings (during safety briefings) of a couple ground incidents involving loss of life. It's not pleasant.

There are CVR recordings which have been released though, although not by the US as per regulations. It's more more harrowing than the final words of someone calling the police before seeing their family killed, or a home video someone makes that catches a horrific accident.

"I've seen a transcript which ends with... " - would that be this one?

Yeah that's the one. Grim. I wonder that they had the presence of mind to say that in the last moments!


text:
Can you jump out of a crashing plane just before impact to improve your chance of surviving?
Let's say a plane is going to crash and all people on board know this.
Why can't they jump from the aircraft when the plane is near the ground?
They would have higher chances of surviving, right?

Higher perhaps, but the improvement would be insignificant: see this question on physics.SE which is the same scenario, and consider that an aircraft would probably start 'falling' from much more than the 10m or 20m described in the some of the answers. And that's without considering practical difficulties like opening the door and fighting off the other passengers who want to jump too.

Your question is extremely closely related to In the event of a crash over thick snow or a forest, would it be safer inside the aircraft or free-falling? - While your question is not about forest or thick snow, the factors at play deciding the fate of your body already seems to be answered in the other topic.

Mythbusters already addressed this with jumping in a runaway elevator.  Buster fared no better jumping--and a plane crash involves a lot more speed than an elevator.

sure, Bugs Bunny has done it lots of times. Seriously,  a case of a british bomber at about 500 feet and the crew ejected. The low altitude, net vector of the forward motion & falling airplane & upward ejection caused one guy to hit the ground still strapped to the seat, the chute never had a chance to deploy yet he skid safely to a stop and walked away uninjured.

Why can't they jump from it when the plane almost reaches the ground?
  They would have higher chances of surviving, right?

Wrong.
Remember your body still has roughly the same momentum it did when it was in the vehicle - just because you jumped out doesn't mean you're not still moving forward at a rate of speed the human body simply isn't designed for.
Let's reframe your question:

If you were in a car doing 60 miles per hour and you knew the car was about to crash into a brick wall, would you jump from the vehicle, or would you remain strapped into your seat where the car, seat, seatbelt, and airbag will all help absorb the impact?

As with a car, being inside an aircraft and properly restrained will afford you substantial protection from the initial impact, and some of the crash energy will be absorbed damaging the aircraft. That increases the length of time it takes to decelerate your body and gives you a better chance of survival.
If you jumped out of your friend's Cessna because it's going down and might hit some trees your unrestrained body would be decelerated far more abruptly when you hit the trees (or the ground).
You might survive, but you've lessened your chances considerably (and the Cessna is a best-case scenario - your forward speed would be around 60mph as in the car example. For something like a 747 you'd be in the 150 mile-per-hour range or faster when you jumped out, which is almost certainly not survivable).

The only time it's safe to jump out of a crashing airplane is if you have a parachute and sufficient altitude to use it.

Unless you're Nicholas Alkemade...
https://en.wikipedia.org/wiki/Nicholas_Alkemade
(Or one of the other people referenced from the same page)

@JonStory 7-to-(some really big number of people who have died from falling off things) odds . . . I tell ya what if my neighbor jumps can I have their seat cushion to help absorb impact in my brace position? :)

@DanPichelman Even that isn't a certainty. Ejector seats are carefully designed to blast you clear of the plane; regular parachute jumps are done at low speed and often from a door at the rear of the plane; test flights of commercial planes have an escape hatch, again designed to guide people clear of the plane. None of these conditions is likely to be met in a situation where they weren't deliberately made to happen. You don't want to jump out of the plane and turn into a very large bird-strike.

@voretaq7 hey I wasn't saying it was a good strategy, just that a few have made it work :p

@voretaq7 just to play devil's advocate, air resistance will slow a person down more than it would slow a 747 down in the time before hitting something solid :)

@hobbs That's true, though it depends on when you jump. (Of course if you jump out high enough that air resistance will substantially reduce your forward velocity you'll now have your freefall velocity to contend with, which is usually enough to have you make a mighty SPLAT when you hit the ground…)

No, jumping out of a plane just before it crashes wouldn't help you, for the same reasons that jumping off the floor in a free-falling elevator wouldn't help you.

But there it says you can reduce the energy by 50%. That's pretty helpful

@why Flaming death minues 50% = flaming death.


text:
Where can I find Air Traffic Control information material suitable for beginners and students?
I would like to learn more about how the Air Traffic Control system works - are there any resources available which describe the system that would be suitable for a beginner or student?

Another good reference would be the training materials for VATUSA. This is part of an online system called VATSIM where people both fly and provide ATC virtually.
Although some of it does not apply to real operations (and of course none of it should be actual operations), it still covers real procedures based on the actual rules.
The basic guides do a good job of providing an introduction to the basic concepts, but the more advanced stuff is less helpful.
Other regions have their own VATSIM organizations, if you prefer a region other than the US.
In addition to the materials available, participation is also great for learning. Through VATSIM you can get training on the materials and then get hands-on experience working with (virtual) traffic.

Good, I hope this will help others too. Thank for your help and sharing useful links with me. Appreciated.

VATSIM is also a good source of practice - some of what you learn isn't applicable to working live traffic,  but much of it is.

I'm a former VATSIM controller with a relative who is real-world ATC in ZTL (Atlanta Center).  I've shown him VATSIM and also visited ZTL with him.  He was impressed by the procedures and phraseology of the VATSIM controllers.  And during my visit to ZTL, while I would never pretend to be able to do their job or even in the same ballpark as the real-world guys and gals, I can say an awful lot was familiar to me. Because of my VATSIM experience, I knew exactly what was going on.

@wadesworld Anecdotally the phraseology of VATSIM controllers (and pilots) tends to be better than what I hear on the radio flying around New York :)

@voretaq7 You're right.  Real-world controllers and pilots are actually a bit more lax about phraseology since they just need to get the job done and have bigger concerns than whether word 4 in the sentence is exactly what is written in the manual.

The first two places I would start would be the FAA's Air Traffic Control landing page, and Chapter 4 of the Aeronautical Information Manual which deals with air traffic control (and is generally the level of information student pilots are expected to be familiar with).
While these references are both for the US/FAA air traffic control system there are strong similarities among the systems in most countries (the ones with the greatest variation would be countries where the military handles air traffic control).

If you're looking for more advanced information pretty much all of the operational and training manuals the FAA uses are available to the public.  These are likely of more interest to someone studying to become an air traffic controller - the information in them is substantially more technical and "job-oriented" than what's presented in the resources above.

Nice one, currently I'm going to be interview soon for an ATC job. Had no experience or so whatever about ATC specifically aviation. So I need help to know a little bit better about ATC and where usually people will go about to learn about ATC in aviation community cause Google only populate the popular site, cause I'd have found some good one through my friends and others. Well thank you very much.

The Air Traffic Basics course is the first information new FAA controllers are exposed to, and it is available to the public. This is a comprehensive introduction to background aviation knowledge from a controller's perspective.
Beyond that and getting into phraseology, there is FAA JO 7110.65. This document is THE standard for U.S. air traffic control procedures. Everything controllers say comes from that book.


text:
Why does the 1090 ES use the same frequency for both uplink and downlink?
The 1090 ES is the transponder system used in ADS-B.
Why does the 1090 ES use the same frequency (1090MHz) for uplink as well as downlink? 1090 ES is an extension of Mode S. The Mode S used 1030MHz for uplink and 1090MHz for downlink. Why did the 1090 ES change that? Having two frequency channels would be better (More space less noise due to message collisions) for communication right?

There is no uplink in ADS-B. The aircraft is broadcasting information which is received by parties unknown to the ADS-B transmitter. There is no reply and there is no uplink like there is in Mode-S.
The receiving parties listen on 1090MHz for ADS-B broadcast.
A technology related to ADS-B is TIS-B (Traffic Information Service - Broadcast). This is used in the USA to transmit positions of aircraft that are not equipped with 1090ES ADS-B but are detected by FAA radar or UAT ADS-B in an ADS-B like format on 1090MHz. This allows parties listening on 1090MHz for ADS-B traffic to identify non 1090ES equipped aircraft with the same receiver. TIS-B could be seen as form of uplink, but uses the 1090MHz frequency nevertheless as it allows the same ADS-B receiver to be used.

what about the TIS-B (traffic info) and FIS-B (weather info) shared by the ATC? Isn't that considered as uplink? If the ATC uses the 1030MHz channel, there would be even less congestion compared to the 1090MHz. That was what I was trying to point out.

@Clive TIS-B is something different than ADS-B. Indeed it causes some congestion on the 1090MHz freq. One of the reason we don't have it Europe. FIS-B is a UAT application, it does not work on 1090ES.

It's a broadcast protocol - devices spontaneously announce themselves for listeners to hear. There are no interrogations to put on a different channel.


text:
Why Rotax 912 "burps the tank"?
The Rotax 912 engine has dry sump forced lubrication system. They say rotating the prop in the correct way (counter-clockwise when looking from the prop side) will get the oil from the bottom of the crankcase to the oil tank.
Questions:

How does this "force back by blow-by gases" work? Is it air which gets through the carburetors and cylinders to do the crankcase what pushes the oil from the engine? If oil gets from the bottom of the crankcase to the top part of the oil tank, where the "burping" sound emerges then? In the oil line, or at the bottom of the crankcase?
If the oil gets pushed by air, why is it bad to rotate the prop in the opposite direction? Air would get to the bottom of the crankcase and air would get to the oil input line and "burp" at the bottom of the oil tank. But if engine then runs in correct direction again, won't the oil be sucked from the bottom of the oil tank again? My only guess is it won't due to the air in the oil pump which would just spin without sucking any oil...

The Rotax 912 engine uses a dry sump, and does not have a scavenger pump to return oil to the sump. Instead it uses pressure escaping past the piston rings to force the oil back into the sump.
The "burp" you hear coming from the (open) oil tank happens when most of the oil in the crankcase has been returned to the tank: the air escaping past the rings bubbles up through the oil tank.
The propeller is pulled through "forward" (in the normal direction of rotation) for a few reasons, mainly that it's the direction that the gearbox & accessories are all designed to turn. Turning in the normal direction of rotation also ensures that oil is pumped through the rest of the system, particularly the valve train.
Reverse rotation can pump air into the system which may cause problems if you try to start/run the engine immediately after. I believe the recommended procedure is to "burp" the engine a few times to re-prime the oil system if it's pulled through backwards.

Thanks for the answer! "the air escaping past the rings bubbles up through the oil tank" - is that really correct? I thought the oil pipe from the crankcase goes to the TOP of the oil tank and that oil "falls" from the top in the oil tank. Oil from the tank to the engine is "sucked" from the bottom of the oil tank.

This "burping" is probably happening while the engine is running too, it just can't be heard due to the engine noise. The burping noise probably emerges at the bottom of the crankcase (not in the tank) when almost all oil has been returned. This burping probably occurs because air pressure from the cylinders creates more oil flow out of the engine than the oil pump for the oil coming in, that's why there is always some oil in the tank (which is good!). That also explains why burping shouldn't be done just after stopping the engine - as we want to let the oil drip to the bottom of the crankcase to get as much oil as possible out of the engine.


text:
How does the airline decide how many meals to load aboard a flight?
I've researched a bit, and found that entire load of meals may be discarded if a flight is delayed. I've also seen some posts that say first/business class meals are stocked at 110% of capacity. Additionally, some posts say that meals are based on the actual passenger count, rather than the seat count. I'm curious as to the actual practice for determining how many meals make it aboard a commercial flight.

I'm not sure this is actually on-topic here. It seems to be a question of company policy rather than aviation, which would seem to make it off topic. It may possibly be on topic on [travel.se].

There are on Travel.SE 2 specific tag that apply to your question when put together (namely air-travel and food-and-drink). Your question is a quite good question to be asked there.

Although to answer the question, they perform some statistical analysis: ie the airline records how many of each meal are used/requested and slowly balance the numbers until you have just enough of each. The 110% stock is due to wanting to ensure that your (profitable!) business class passengers don't go away disappointed, and always have the choice. Economy class tends to carry a few extra of each, but will sometimes run out as they don't carry as large a buffer


text:
What are the protocols of flying over conflict areas?
Apparently Malaysia Airlines Flight 17 used a general airway over the east of Ukraine, which currently is 'in conflict'.
What are the 'normal' procedures regarding (not) flying over conflict areas? Who is responsible for an up-to-date risk assessment? Who advises? Who makes the (final) decision?

Here's an op-ed in the New York Times you might be interested in: http://www.nytimes.com/2014/07/19/opinion/malaysia-airlines-flight-17-the-ukraine-wars-victim.html?hp&action=click&pgtype=Homepage&module=c-column-top-span-region&region=c-column-top-span-region&WT.nav=c-column-top-span-region&_r=0

BBC: Who, what, why: How often do planes fly over conflict zones?

This is horrendous Incident, and my heart goes out to the victims Families who lost their loved ones.

The FAA filed a special Warning flying over Crimea back in April but apparently not where the airplane went down.
The procedure is to avoid flying over (Dead/No fly Zones) which Aviation Authorities are responsible in providing to the Airline companies so they can adjust their flight plans.
FAA Warning over flying over Crimea.

Planes Flight Path.

Did the EASA publish a NOTAM for the area as well?

Yes, on the same date (3rd of April) --- http://www.aviationnews-online.com/regulatory/easa-issues-crimea-safety-warning/

Note that the above FDC NOTAM concerns the Crimean peninsula and surrounding airspace, and does not cover the region of current conflicts in eastern Ukraine.

@casey -- The Route was closed @ FL320 but the plane was @ FL330 -- by the looks of it there was plenty of traffic on that route -- http://www.wired.com/2014/07/the-crashed-malaysia-airlines-plane-wasnt-the-only-jet-in-ukraine-airspace/

@casey Sure but the question is about normal procedures for (not) flying over conflict areas. The NOTAM quoted is an example of such normal procedures.

authors: К. Келлогг Смиф


text:
What level of liability should a GA renter's insurance policy have?
As a GA pilot who rents planes occasionally, I am looking at buying a renter's insurance policy so that I'm covered against the injury of any passengers as well as damage to the aircraft.
For bodily injury, I based the amount off of my car insurance. That is, in the neighborhood of $100,000/person. Is that assumption correct?
What I don't know is what level of aircraft damage liability is an acceptable amount. I mostly fly Cessna 152s, 172s, and Piper Warriors. Of course, it isn't worth paying for a liability that is more than the plane is worth. So what would be a good level of liability to be covered for?
Further, I understand that an FBO will have insurance on its planes. However, this insurance is to protect the FBO, not the pilot. Should the FBO's insurance affect the level of insurance I, as a renter, choose?
For reference, here are Avemco's current annual damage liability rates for single engine planes:
Limit      Premium
-------------------
$1,000     $60
$5,000     $120
$10,000    $155
$15,000    $210
$20,000    $220
$25,000    $275
$30,000    $310
$35,000    $365
$40,000    $415
$45,000    $465
$50,000    $500
$55,000    $540
$60,000    $560
$65,000    $605
$70,000    $675
$75,000    $725
$80,000    $745
$100,000   $925
$150,000   $1,400

A key component to liability insurance is the deductible.  The higher the deductible, the cheaper the insurance.   Obviously, if you take a $5K deductible, than a $1,000 and $5,000 policy don't even exist, and the $10K policy should be very cheap.

This question isn't possible to answer definitively because it depends on your risk tolerance and ability to pay cash yourself directly if needed; you should really consult an insurance professional. Details of coverage vary widely depending on the policy and the interaction between the FBO's insurance and yours could be relevant or not, depending on their policy. I think the general opinion is that renters should have insurance (I do) but how much to have and the specific details of the contract are individual decisions.


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How does the aviation industry train and certify English proficiency?
How does the aviation industry (pilots and ATC in particular, as opposed to cabin crew) train and certify English proficiency? Do they use classes and examinations associated with general-purpose English training, or ones specifically associated with the aviation industry?
I heard several years ago from Air Crash Investigation that a contributing factor for the Avianca Flight 52 crash was the use of the wrong terminology in communication between the non-native speaking pilots and the ATC.

Not relevant, but what prompted me to ask the question: Berlitz ad The German Coastguard

Proficiency in English is something very much required in aviation. One other major accident I remember is the Charkhi Dadri mid-air collision, which resulted in 349 fatalities, and one of the major reasons was believed to be the poor English of Kazakh Airlines crew.
Keeping such importance of English in mind, and the need to standardize English testing and training, the ICAO  has issued Guidelines for Aviation English Training Programmes.
Training
Candidates are trained for Aviation English in a very similar way to it would be for any other purpose, other than ensuring the following points:

Use of operationally relevant, work-related language.
Exposure to an aviation environment.
Practical training activities such as Listening practice in ATC lab.

Testing
The Test of English For Aviation is a test Developed by Mayflower College in accordance with ICAO's ratings.
Here is how the candidates are tested in TEA:
TEA candidates can score from Level 1 to Level 6 but most candidates score Level 3, 4 or 5. Candidates are awarded a mark from 1 to 6 for each of the 6 skills: pronunciation, structure, vocabulary, fluency, interactions, comprehension.

Read this document

There exists specialised "Aviation English" training for non-native speakers. In such courses, they teach not only basic English proficiency, plus a large amount of technical aviation-specific language (both common and mandated by ICAO standard procedures).
Here is one organisation that offers such courses: Airways Aviation English Training.


text:
Why did the manifold pressure gauge on the Ki-61 have negative values?
The manifold pressure gauge of the Japanese Ki-61 has values which go from +40 to -40. I know this has to be relative to something since negative pressure can't exist (you can't get more vacuum than a vacuum), but I'm not sure what it's relative to.
Why did the manifold pressure gauge on the Ki-61 have negative values?

Just a guess, but perhaps it's relative to static pressure?

Disclaimer: I do not know if my answer is correct, but I think it is plausible.
The Ki-61 engine was the Kawasaki Ha-40, a Japanese version of the Daimler-Benz DB 601 Aa turbocharged inverted-V inline engine, so I would assume that the engine instrumentation was inspired by what was used in Germany at that time. Manifold pressure was measured in ata (Atmosphäre absolut, close to 1 bar), which is multiples of the standard atmospheric pressure. The picture below shows the manifold pressure gauge of the Me-109 G, which used the DB 605 A. This was derived from the DB 601, and the manifold pressure gauge has a range from 0.6 ata to 1.8 ata.

The DB 601 A had a maximum manifold pressure at sea level of 1.35 ata. This lets me assume that the Ha-40 was also turbocharged to 1.35 ata, which is 1.368 bar. The critical altitude was probably only 3700 m. The regular DB 601 A had a critical altitude of 4500 m, which was reduced in the DB 601 Aa export version to 3700 m. I could not find a direct reference for the critical altitude of the Ha-40.
At the 11,600 m service ceiling of the Ki-61, the atmospheric pressure is 206.6 mbar. Now we need to factor in the ram pressure gain of approx. 20% at 300 kts TAS, and with the 2.14 compression ratio needed for a critical altitude of 3700 m, this would translate to a manifold pressure of 530 mbar or 0.522 ata. If the Kawasaki engineers raised that to the 2.37 compression ratio of the original DB 601 A, the manifold pressure at 11,600 m could even had been 578 mbar or 0.570 ata.
The most plausible explanation for the numbers on the manifold pressure gauge now would be percentage over rsp. below 1 ata, the static pressure at sea level. Since it is more important to not overstress the engine at low level, the +40 upper limit fits well to the maximum 1.35 ata setting. This fits also well with the static value of 0, meaning 1 atmosphere. At high altitude the compressor would run at maximum setting anyway, and the scale here is for informing the pilot about available power, and would not cover the lowest values close to maximum altitude.
I could not find a performance chart for the Ha-40 or the DB 601 A, and the nearest Google dug out for me is the performance chart of the DB 601 E below. Please note that this engine had more performance (2700 RPM and 1.42 ata manifold pressure for takeoff; the DB 601 A used 2400 RPM and 1.35 ata) and a higher critical altitude of 4800 m:

The Zero value is the standard reference point for Japanese Manifold Pressures which is 750 mm Hg Absolute Pressure.
The Ki-61-I series aircraft was equipped with the Ha-40 engine which was not an exact copy of the DB 601Aa but derived from it.
The normal maximum Manifold Pressure was +240 mm Hg which would be
240 mm + 750 mm ==> 990 mm Hg
or 38.98 inches Hg on a more familiar scale.
The maximum Manifold Pressure available for Take-Off (1 minute limit in the original DB 601Aa but not known in the Ha-40) was +330 mm.
So.... a scale of +- 40 centimeters makes pretty good sense for a Manifold Pressure Gauge.

Ivan.

Almost Forgot: Critical Altitude of the Ha-40 was 4200 meters.


text:
Is pressure pattern navigation used in the real world? How effective is it really?
Pressure Pattern Navigation is a method of capturing favorable tailwinds when flying long-distance trips.
The basic methodology can be found in this PDF.
I imagine that airline captains aren't all that free to choose their routes (correct me in the comments if I'm wrong!), but this seems to be perfectly suited for the corporate and charter crews among us. However, I've never heard of any crews using it in real life.
Does pressure pattern navigation work in the real world? How effective is it?

Airline crews don't do the route planning. We leave that up to dispatch and only intervene if there is an issue with the route (wx, etc)

It is used in the real world, especially by airlines. The best example is the North Atlantic Track system. The tracks are tailored every day to maximize the tailwind on West to East crossings and minimize the headwind East to West crossings.


text:
Why the pitot tube is heated but the static port isn't?
In some light aircraft, like a Cessna 172, the pitot tube is heated but the exterior static port isn't. The usual reason I've heard is that static ports are much less susceptible to icing, but why is that the case?

You'll find your answer if you take a look at the form of a pitot tube or other kinds of ducts where pitot (or total) pressure is taken and compare it to the parts of the aerofoil where ice is prone to accumulate. Ice will most likely form on the leading edge of thin surfaces in the airstream but not on the side of the fuselage. If ice would built up there you will probably have some more urgent problems then switching to your alternate static source ;)

Some aircraft DO heat the static port, if only by side effect: Piper PA28 aircraft (Cherokee/Warrior/Archer/Arrow ) for example have the static port located on the pitot mast.  When pitot heat is turned on in these aircraft the entire mast heats up, including the static port.
This characteristic would be the same on most aircraft with a combined pitot/static mast.
As for why most light aircraft don't heat the static port, there are two big reasons.
First, as you've heard, static ports are normally not located in areas susceptible to icing -- they're on the side of the fuselage in areas of relatively undisturbed air (on most of the Cessnas I'm familiar with they're forward of the doors, on other aircraft they're right around where the registration numbers get painted). As Falk pointed out this means ice normally won't accumulate there.
As an additional protection some aircraft have multiple external static ports (connected to a shared static line) so that if one port does get iced over you still have a source of reference pressure.
Second, a static failure is relatively easy to deal with - particularly in unpressurized light aircraft.  Many unpressurized aircraft that are regularly flown in instrument conditions will have an alternate static source in the cabin (which is simply a valve that opens the static line to cabin air), or the traditional "break the VSI glass" solution can be used to get a mostly-working altimeter.

Because static ports are much less susceptible to icing, exactly as you said.
(Also because alternate static sources are available.  If your static port ices up, you can pull the lever for alternate static source, and get another air-source from inside the cabin)

You didn't actually answer the question ("why is that the case?").

To perhaps be a bit more clear on the "why" question, the opening of pitot tubes must face into the windstream. By contrast, the opening of the static ports should be perpendicular to the wind stream. Since the water that causes ice accretion is generally moving with the air, it will be constantly hitting the pitot tube opening, but not hitting the static port opening (or not nearly as much, at least.)
It is not by coincidence that pitot tubes are generally in locations where ice will accrete while static ports usually aren't. It is instead a necessary consequence of each of them meeting their design goals.

The first spots where ice accretion occurs are sharp angles and spikes.  This makes the external thermometers and pitot tubes the first locations where icing occurs.  Ice doesn't normally accrete where the static ports are located unless you have a huge icing problem.


text:
Can a person with Type 1 Diabetes get a medical and learn to fly?
A friend of mine has had Type 1 diabetes most of his life, and has it well under control.  How does he go about finding out if he can get his medical and learn to fly?

if not limited to US you might check out link

@arcfrostt Thanks, but this question is tagged faa-regulations.

As far as the FAA is concerned your friend can get a medical, but he will need a special issuance medical, will only qualify for a third-class medical, and will be prohibited from operating aircraft outside the United States.
In addition there are some specific requirements to obtain the special-issuance medical for insulin-dependent diabetics, and monitoring requirements before and during flight which must be adhered to.
The FAA has a page of guidance for Aviation Medical Examiners that deals with insulin-dependent diabetes which your friend should review (and be prepared to discuss with their AME).
Diabetes.org also has a page on Diabetes and FAA Certification, and AOPA's medical folks would be a useful resource to help ensure that the special issuance medical goes through without any hiccups.

I haven't reviewed all of the links that you posted, but can all Type 1 diabetics be approved, or are there conditions?  (A list of qualifying/disqualifying conditions would be good here for future visitors!)

Things change.  Recently an airline captain who is insulin dependent, flew with a First Class special issuance medical.  At this moment, there are six airmen in the US who have First Class special issuance for insulin dependent diabetes.
I suggest that your friend may want to re-investigate his obtaining a medical.
The guidance for third class is here:
https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/dec_cons/disease_prot/diabetes_insulin/initial/
Continuous Glucose Monitoring (CGM) special issuance is available for all classes of medicals:
https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/dec_cons/disease_prot/diabetes_insulin/initial/
CGMs are devices which have a sensor on the body which reads the glucose level, and the patient / pilot can adjust the insulin they get based upon readings from the monitor.  Insulin can be added by injection or by an insulin pump.  Many CGM will sound an audio alert and even send messages, should a patient have declining blood glucose.  The primary safety of flight concern is that low glucose can cause loss of consciousness and can impair judgment. Utilizing CGM devices mitigates that risk, and the FAA has refined their approach to diabetes, most notably with respect to First and Second Class medicals.
The new US FAA policy reflects what some other countries have had in place for some time.

The main problem in Type-1 DM is that anything that increases the body Glucose consumption may yield a potentially obtunding and sometimes deadly too low blood Glucose levels (Hypoglycemia in medical terms), this may come from a physical exercise while on the usual Insulin dose, and also from the mental stress from piloting, that may increase the Glucose consumption and lead to the catastrophic event of a loss of conscience, a blackout, or death while in command of an airplane. Automated Glucose monitoring devices that act mimicking the Pancreas islet cells function, measuring in a continued real time blood glucose levels, and releasing Insulin to the blood in accordance, are under strong research by many, but as in both T-I and T-II DM, (DM may be considered a mainly self-managed disease), the race is against the long term complications from poor, too high or too unsteady blood Glucose levels control, a too low blood Glucose level is an immediate life-threatening situation. People suffering T1DM must acknowledge that some activities are too dangerous for them and in certain circumstances for those close to them, at least in the current state of the medical arts, and thus, T1DM patients better refrain from engaging in some types of risky activities. Acting as an Airplane pilot is one of it.

Very little of this answer is actually answering the question, and your answer seems to contradict the FAA source voretaq7 cites. Do you have any evidence that it's unsafe to specifically fly aircraft?

It may contradict voretaq7, but Everything he says is true, however.


text:
Is the main landing gear of a large airplane designed for sideways (crabbed) touchdowns?
Is it just my imagination, or is it a fact that many large airliners actually touch down "crabbed" on difficult crosswind landings?
Here's what I mean in this video, or as shown in this picture:

Is the main landing gear specifically designed to allow this? Is it recommended or discouraged by the manufacturer?

Related question about landing techniques & crab-vs-no-crab

I enjoy watching crosswind landings way more than a reasonable human being should.

As I remember from initial ground school for the 747-100 and -200 in 1990, the recommendation was to simply touch down in a crab. In actual practice we typically used a combination of slipping and a crab at touchdown, how much of each depended on the pilot. In all but the heaviest xwinds you could take out the crab. I forget the max amount of wing down you could go, but I seem to remember that 5 degrees of wing down was fine, and that would get a lot of slip.

@Terry: It must take some skill to time the straightening up correctly.

@copper.hat It's a matter of timing, and it comes with a little practice. It's better to err on the side of not getting it straightened out before touchdown as all that means is you're touching down in a crab. If you get it straightened out too soon, the xwind will drift you sideways with respect to the runway. If that happens, put the crab back in and, possibly, more bank. It's not all that hard to time it well in a 747 or any aircraft that you're getting altitude calls from the radar altimeter or another crew member..

@CameronMacFarland agreed, they are my "kitty videos" as well; well those and Russian Car Crash vids. :D

The landing gear is indeed designed to cope with crabbed crosswind landings.
The recommendation is to avoid crab on landing however in severe crosswind conditions it is sometimes impossible to decrab completely without introducing excessive bank. Therefore some residual crab has to be allowed.
Airbus recommends less then 5 degrees of residual crab on landing, but aircraft are designed to cope with more.
An exceptional design is found in the B-52, where the main landing gear can rotated to be aligned with the direction of travel to deal with extreme crab angles.

Nice. The 747 in the picture above confirms that it can be a lot more than 5 degrees :-)

That's an All Nippon 747, as a Japanese Airline, they're into their drifting.

I was going to mention the B-52's rotating gear; good catch there. I wonder if any other designs have incorporated it. I know the 747's mains will rotate counter to the nosewheel to assist in taxiing on the ground, but if you were applying rudder to straighten out of a crabbed approach at touchdown, that system if active would point the wheels into the wind instead of down the runway.

@PhilippeLeybaert this is not obvious. The aircraft may be going to the left of the runway, the angle of view may emphasize some angle and reduce other, and lots of more perception biais may exist.

All Airbus aircraft have high clearance below the engines, so they can afford recommending lot of slip. Especially B737 has low clearance, so crabbing is recommended for it.


text:
Why is volcanic ash hazardous?
What happens on the engine after ingesting volcanic ash? Is volcanic ash dangerous for the cockpit crew too? Despite the ground stations, is there any other way to identify volcanic ash while flying?

The British Airways Flight 9 incident will answer pretty much all your questions in real life format https://en.wikipedia.org/wiki/British_Airways_Flight_9

Or KLM flight 867

Or this NASA DC-8

Kinda makes you wonder, between dispatch, the pilots and ATC how can they NOT KNOW a volcano is erupting on their flight path?

Ash has no radar signature, it must be visually identified and while they may know at the site it may take some time for that information to get to the correct parties and disseminate out. In the air the pilots may see it as heavy cloud cover or simply continued IMC if they are already in IMC.

As mentioned in the comments BA flight 9 is a thorough example of why ash can be a huge issue.
Relevant points of that incident
One issue that ash creates is it cannot be seen on radar so it can be hard to track at night and must be reported by other means. This is deceiving as you would physically see what appears to be a cloud yet weather radar would not show anything.
Inside the cabin, this depends on the granularity of the ash. Some of it will be filtered by the plane's air filters as any particulate matter would. However, ash can be extremely fine and what does make its way through the filters will present itself in a smoke like manner. This may cause alarm in the cabin and a false indication of a fire.
When it comes to the exterior of the plane there are a few issues. The fine grains hitting the plane at that speed will act like sandpaper. The windshield may become scratched to the point you can't see through it (which is not actually as big of an issue as some may assume). The paint will be effectively sanded off which in the short term is not a huge issue. More importantly it could affect your instrument inputs by clogging things like the static ports and pitot tube creating erroneous readings in the cockpit.
The engines were the main issue that arose on BA 9. In short the ash entered the engine, was melted and began to adhere to the blades of the jet. In turn this blocked the engine airflow and caused an engine stall. In the process BA 9 lost all four engines; however, when they began to descend the solidified ash cooled and broke off the engine allowing a restart.
Air crash investigation covered the BA incident nicely in season 4. You can watch it here.
Other risks that BA 9 did not have
For a piston plane ash could potentially clog the air filter and starve the engine of air. Any super-fine ash that could get through the filter runs the risk of being ignited and causing detonation issue with in the engine. Likewise the sanding/grinding issue could have adverse effects on the propeller shape reducing its efficiency or even overall ability to act as an airfoil.
There is a nice wiki on it here and it appears that as of 1991 centers called "Volcanic Ash Advisory Center" have been set up to help track these very issues.

To start, because it's not just ash.  Larger particles can be carried in the plume as well.  In addition, the intense pressure and temperature of jet engines can reform the debris into larger particles and cause internal damage.
Wikipedia on the 2010 Eyjafjallajökull eruption, which is a likely source of this question.
There is also hazard of external metal and glass damage due to high speed impact with these particles.
Finally, visibility is obviously affected and lightning is a common occurrence in the plumes.
It's bad all around.